The music style you are mostly going to listen to as each manufacturer of headphone tends to have a different sound signature. For example Grado headphones are great for rock and metal as they have real bite on the highs like vocals and drums.

and also the enviroment your are likely to use them within the most. There's not much point in using open back cans when commuting on the bus or train as your are going to hear all the sounds of your enviroment and people around you can hear your music, which could be embarrising depending on your taste!! = )

if you would like to have expert advice on headphones a good idea is to give hifiheadphones a call or a mesage.